Stress corrosion cracking is a phenomenon that occurs in certain materials when placed in a stressed state and exposed to a corrosive environment. The stressed state may include a state wherein the material is subjected to tensile stress. Tensile stress may include residual stress occurring in the material as a result of a manufacturing process such as machining, welding, and/or heat-treating the material. Tensile stress in the material may also result from an externally-applied load such as a sustained load or a cyclic load applied to the material in its normal operating environment.
Corrosive environments that can lead to stress corrosion cracking include environments where the material is exposed to water vapor or an aqueous solution. The occurrence of stress corrosion cracking is highly dependent on the chemical makeup of the environment and the composition of the material. Materials that are susceptible to stress corrosion cracking include metallic materials (e.g., aluminum, stainless steel and other ferrous alloys), polymeric materials (e.g., polycarbonates, polyesters), and other materials. The occurrence of stress corrosion cracking is also highly dependent on the temperature and relative humidity of the surrounding environment. Stress corrosion cracking may occur as intergranular cracks that penetrate from the surface into the depth of the material. The cracks may be microscopic in size making detection difficult. In addition, the surface of the material may appear to be unaffected which may increase the difficulty of detecting stress corrosion cracking.
Testing a material for susceptibility to stress corrosion cracking may be performed at an accelerated rate under a set of testing requirements published by the American Society of Testing Materials (ASTM). ASTM G-44 provides for accelerated testing of metallic materials by alternate immersion of a test specimen in a 3.5% NaCl solution. ASTM G-44 includes relatively stringent requirements for controlling the temperature and relative humidity of the environment to which a test specimen is subjected, and for controlling the volume of the corrosive solution relative to the test specimen surface area immersed in the corrosive solution. ASTM G-44 additionally provides for the option of inducing tensile stress in a test specimen during alternate immersion testing. Tensile stress may be induced in a material up to a percentage of the material yield strength, and which may be representative of the stress to which the material may be subjected in its service environment.
The prior art includes several methods for alternate immersion testing of a material for evaluating stress corrosion cracking One prior art method uses a Ferris wheel arrangement having trays containing test specimens which are rotated through a corrosive solution. Unfortunately, the Ferris wheel arrangement lacks the ability to control the immersion time period of the test specimens independent of the drying time period due to the dependence of the immersion and drying time period on the depth of the solution and the rotational speed of the wheel. If the rotational speed of the wheel is decreased to provide a longer immersion time period, then the drying time period is increased. If the solution depth is increased to provide a longer immersion time period, then the drying time period is reduced. Another prior art method provides a means for alternate immersion testing of test specimens. However, the method lacks the ability to control the testing environment in accordance with the stringent temperature and humidity requirements of ASTM G-44.
As can be seen, there exists a need in the art for a system and method of alternate immersion testing of a test specimen for stress corrosion cracking susceptibility which provides a means for altering the immersion time period independent of the drying time period. In addition, there exists a need in the art for a system and method for alternate immersion testing of a test specimen for stress corrosion cracking susceptibility which provides a means for controlling the temperature and humidity of the environment to which a test specimen is subjected.